Field of the Invention
The invention relates to a circuit configuration for digitally setting analog parameters.
Such circuit configurations are used, for example, for digitally setting filter parameters of an analog filter or for digitally setting the gain of an analog amplifier.
A book entitled "Electronic Circuits--Design and Applications", by U. Tietze and Ch. Schenk, Berlin, Heidelberg 1991, pages 399 to 402, discloses the use of a digital/analog converter having a resistor network for digitally setting respective parameters. The digital/analog converter takes the place of a corresponding resistor element producing the respective parameter. U.S. Pat. No. 3,309,508 describes a circuit configuration for multiplying two analog voltages, in which the first of the two voltages is converted into a digital representation before multiplication. Each individual bit of the digital representation switches a current switch having an input to which the second analog voltage is connected. The current flowing out of the current switches is then weighted through the use of resistors. Each resistance value corresponds to the weighting of the respective bit and has to be set exactly for a precise result.
However, resistors integrated in integrated circuits have a wide component tolerance, as well as a considerable temperature response, so that they can only be used to produce very noncritical configurations, or else they require considerable additional complexity in order to achieve sufficient accuracy and constancy. Higher-order digitally adjustable filters with a relatively high degree of accuracy therefore require comparatively high circuit complexity.